Because I don't see other use cases besides using string as an iterator. toFunction is pretty easy to implement and do you really think it's worth adding another String.prototype method (if toIterator is enough) ? : )

Oliver Steele's Functional Javascript library offers automatic conversion of strings to functions, and not just for iterators. For arguments, one can either use Haskell-style parameter lists or implicit parameters. The syntax also allows for more of a pointfree style. For example:

map('1+', [1,2,3]) → [2, 3, 4]

It's actually pretty clever.

Anyway, if this is the kind of feature that Prototype wants to include, Steele's library would seem to be a good starting point.